The present invention relates to a three-dimensional particle image velocimetry method in which a stream system containing light-scattering particles is continuously exposed to a laser lightsheet over a certain period or at least at two discrete points in time and the scattered light from the stream system is evaluated.
Such three-dimensional particle image velocimetry methods applied to droplets have been proposed using different approaches, the primary problem being measurement of the third velocity component in the direction of observation, i.e., perpendicular to the lightsheet. The most common approach uses stereoscopy as described in xe2x80x9cStereoscopic Particle Velocimetryxe2x80x9d by M. P. Arroyo, C. A. Greated, Meas. Sci. Technol., 2, 1181-1186, 1991. The depth information is obtained by evaluating the light emitted by the lightsheet from two directions.
Another approach described in xe2x80x9cDetermination of the third velocity component with PTA using an intensity-graded lightsheetxe2x80x9d by F. Dinkelacker, M. Schxc3xa4fer, W. Ketterle, J. Wolfrum, Exp. Fluids, 13, 357-359, 1992, uses an intensity-coded lightsheet in order to determine the position of the light-scattering particle from the brightness of its image.
Another option investigated by Ch. Brtxc3xccker in xe2x80x9c3-d PIV by spatial correlation in a color-coded lightsheet,xe2x80x9d Exp. Fluids, 21, 312-314, 1996, uses a plurality of differently colored lightsheets that are slightly offset with respect to one another in order to determine the third component of the spectral composition of the scattered light.
Some degree of success was achieved with each of these methods, but the measuring accuracy for the third velocity component was clearly inferior to the accuracy achieved for the components in the lightsheet plane. In most applications, the absolute velocity is lowest in the direction perpendicular to the lightsheet, so that a higher accuracy is desirable in this direction.
The same velocity measurement accuracy in all three dimensions, i.e., along all spatial axes, has been attainable only via observation from two directions perpendicular to one another, which is, however, associated with a high cost.
An object of the present invention is to provide a method with which all velocity components can be determined with essentially the same accuracy using a single direction of observation.
This object is achieved according to a method in which, in order to measure a third dimension along the direction of observation, the phase information of the scattered light is evaluated using a reference wave. With this method, the velocity component of the third dimension perpendicular to the lightsheet can be determined with interferometric accuracy.
In one advantageous method, the scattered light is holographically recorded in order to make it accessible to observation when the flow velocity is high. In evaluating the scattered light or the holographically reconstructed scattered light using laser light having a single wavelength, unambiguous measurement is obtained if the interval between exposures or, in the case of continuous exposure, the time period, is selected to be shorter than the time period in which the expected maximum path traveled by the light-scattering particle in the third dimension is smaller than the wavelength of the laser light.
Practically any desired range of unambiguousness can be obtained with respect to the velocities to be measured in the third dimension by performing two measurements with laser light having two or more different discrete wavelengths and by determining the velocity in the third dimension by the principle of multiple wavelength interferometry on the basis of a synthetic wavelength formed from the other two wavelengths. As an alternative, the reconstruction wavelength can be changed continuously.